1. Field of the Invention
The invention relates to light emitting devices and particularly to current mirrors thereof for heavy loading.
2. Description of the Related Art
FIG. 1 is a chart showing drain current iD of an NMOS transistor, dependent on VDS and VGS, where VDS represents voltage difference between drain and source of the NMOS transistor and VGS represents voltage difference between gate and source of the NMOS transistor. Vt represents threshold voltage of the NMOS transistor. When VDS<(VGS−Vt), the NMOS transistor operates in triode region and the drain current of the NMOS transistor (iD) equals
      1    2    ⁢      μ    n    ⁢      C    ox    ⁢                    W        L            ⁡              [                              2            ⁢                          (                                                V                  GS                                -                                  V                  t                                            )                        ⁢                          V              DS                                -                      V            DS            2                          ]              .  When VDS>(VGS−Vt), the NMOS transistor operates in saturation region and iD equals
      1    2    ⁢      μ    n    ⁢      C    ox    ⁢      W    L    ⁢                    (                              V            GS                    -                      V            t                          )            2        .  As shown in FIG. 1 and the formulae, the drain current of the NMOS transistor (iD) increases with the voltage difference between the gate and source of the NMOS transistor (VGS).
FIG. 2 shows a conventional current mirror, comprising two NMOS transistors 202 and 204 having the same voltage difference between the gate and source (VGS), the same charge carrier mobility (μn), the same gate oxide capacitance per unit (Cox), and gate width to length ratios (W/L) in a ratio of 1:N. The drain and gate of the NMOS transistor 202 are connected to operate in a saturation region. The current through the NMOS transistor 202 is I. The NMOS transistor 204 must operate in the saturation region to ensure load current (IL) is N times the current through the NMOS transistor 202, IL=N·I. The current mirror 200 provides a potential (VDD−VDS) for the load 206. The voltage difference between the drain and source, VDS, of the NMOS transistor 204 should be finite to provide sufficient potential for load 206. As shown in FIG. 1, to operate in saturation region with low voltage difference between the drain and source (VDS), the voltage difference between the gate and source (VGS) of the NMOS transistor 204 must be very low, such that current through drain (iD) is correspondingly low. To provide large load current IL, a conventional solution increases the size of the NMOS transistor 204. However, with current trends favoring small ICs, the increased size of transistors is problematic. A novel current mirror for heavy load (large load current) providing sufficient potential for the load is called for.